Air brake systems are frequently employed in heavy duty vehicles such as tractor trailers and the like. In such systems the brake system is activated by pressurized air transported through tubing. Metal tubing can be employed, however nylon tubing is generally preferred and specifically polyester reinforced nylon tubing. Nylon tubing such as disclosed in Brumbach, U.S. Pat. No. 3,062,241 has met with exceptional success and has been established as the industry standard.
This product includes a nylon 11 or 12 inner and outer layer separated by an intermediate polyester fiber layer. The inner and outer layers are laminated securely together and do not delaminate under operating conditions typically up to 150 psi over a wide temperature range i.e., from 40 to 200xc2x0 F. Further nylon 11 and nylon 12 are well suited for automotive and truck applications. Their resistance to cracking and their fuel and water resistance are exceptional. Further they are not sensitive to stress cracking from zinc chloride as is nylon 6.
Nylon 11 and 12 present some concerns. In particular the cost and availability of these products is a concern. Nylon 11 is only manufactured by one company. Nylon 12 is made by four. Further nylon 11 and 12 are unsuitable for use in temperatures below xe2x88x9240xc2x0 F. or extremely high temperatures i.e., higher than 200xc2x0 F. In Europe, if resistance to higher temperatures is required, manufacturers typically utilize nylon 6. This is less expensive than nylon 11 and 12. But at low temperatures i.e., about xe2x88x9240xc2x0 F., the unmodified nylon 6 tends to be brittle in cold impact.
Nylon 6 is also subject to stress cracking when contacted with zinc chloride and loses physical properties when saturated with water. Zinc chloride resistance is a problem in certain environments particularly where road salt is used to melt ice and snow or near the ocean. Many hose connectors are zinc plated and when contacted with sodium chloride from salt spray and the like, form zinc chloride which causes nylon 6 to break down. For this reason nylon 6 has not been accepted for use in the United States and is only used in limited applications in other countries.
Further nylon 6 per se is not compatible with nylon 11 or 12 and thus forming a laminated tubing using layers of nylon 11 or 12 is problematic. Until now because of the application requirements and compatibility requirements, laminated tubing from nylon 6 and nylon 12 or nylon 11 have not been feasible.
The present invention is premised on the realization that laminated reinforced nylon air brake tubing can be formed from layers of nylon 11 or 12 and nylon 6 or nylon 6-6 by incorporating an intermediate tie layer and at the same time utilizing modified nylon 6 or nylon 6-6. In particular the tie layer for use in the present invention is a nylon 6-12 or a maleic anhydride modified polyolefin which is compatible with nylon 6, nylon 6-6 and nylon 11 or nylon 12 and in particular compatible with even the modified nylon 6 or nylon 6-6.
By utilizing a modified nylon 6 or 6-6 , the disadvantages of nylon 6 are reduced and at the same time it remains compatible with the tie layer. By utilizing inner and outer layers of nylon 11 or 12, the threats to nylon 6 from environmental concerns including moisture and zinc chloride are eliminated and further this is all accomplished while retaining the high temperature advantages of nylon 6 or 6-6.